I’ve had the entire regulator out cleaned it all up and lubricated it. The glass is firmly held in its rubber and metal base frame, so loose glass isn’t the issue, the motor works fine and window goes down about 2 inches then jams.

With a bit of downward pressure from the top of the glass and the switch pressed the window goes all the way down but it doesn’t want to do this without help

The fact that it does go down and up with help suggests to me it’s an adjustment that’s needed, but I cannot figure It out

Based on my W123 Coupe experience:
Some times the regulator mechanism bends a little and causes the window to get stuck on its way down.
The fix is to replace the regulator mechanism.
Hope this helps,
Tony

Life is a journey, with problems to solve, lessons to learn, but most of all, experiences to enjoy.

Jason,
I had this exact same problem on my 1973 450SLC. It only occurred on the right side but had the same symptoms as yours. It would go down a couple of inches then jam. It initially looked like a lubrication problem - the whole mechanism would twist and jam.

I spent ages on this. Removed the mechanism many times, lubricated it, straightened it, compared it to the working left hand side - but no success. It seemed like a geometry problem and I re-drilled some holes in the glass holding bracket to move the pivot back. This improved it, but it still jammed occasionally.

In desperation I sourced a mechanism from a more recent vehicle (I think it was a '77) and the more recent part was quite different with different pivot points. It actually looked much more flimsy than the '73 mechanism but it worked first time, every time. I have no idea why the '73 mechanism stopped working (and why the left hand one still works) but it is obviously marginal. Mercedes must have recognised this and changed the design.

BTW I did not buy a new motor, the '73 motor was fine.

The guide bars were also different. The '73 model was aluminium and roughly followed the curve of the car skin, the '77 was steel and straight.

So, if you can find a later replacement I'm sure that will fix your problem.

Cheers
Pete

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I bought a pair of 77 SLC electric window regulators on eBay shortly after your last post - they arrived today

I removed the drivers side, swapped the window motor over (3 bolts) and refitted the replacement regulator, and as you’ve said it worked perfectly first time! Your advice has saved me ( and I’m sure many others to come ) countless painful hours trying to get these rear windows to work as they should.

Anyone else out there with sticking juddering, half up half down rear windows - this is the fix right here!